1. Field of the Invention
The invention relates to scanning pulsed laser systems for optical imaging.
2. Description of the Related Art
Dual pulsed laser systems comprising two modelocked lasers operating at two slightly different repetition rates f1 and f2, such that δ=|(f1−f2)|<<f1 and δ=|(f1−f2)|<<f2, are useful instruments for the rapid interrogation of optical response functions of widely varying electronic and opto-electronic devices such as photoconductive switches and integrated circuits. Additionally the use of dual pulsed laser systems has also been suggested for THz imaging as disclosed in U.S. Pat. Nos. 5,778,016 and 6,396,856 to Sucha et al.
The use of dual modelocked lasers can be replaced for probing the optical response functions by implementing dual electronic circuit systems, as has been suggested in U.S. Pat. No. 5,748,309 by van der Weide. The approach has some benefit for the interrogation of the spectral dependence of signal transmission in the THz spectral range. Two pulsed signal sources also operating at two slightly different repetition rates f1 and f2 were disclosed, which produce emission in the THz spectral range made up of frequency lines comprising pure harmonics of the two repetition rates. Detection of the beat signal at δ, 2δ, . . . nδ is then used to infer the signal transmission at the harmonics of the repetition rate f1, 2f1, . . . nf1. Note that in this scheme a beat signal at a difference frequency as low as δ is used, which is not ideal, since δ typically has a small value where acoustic noise can corrupt the signal.
The use of mode locked lasers was again later disclosed by Keilmann et al., in ‘Time domain mid-infrared frequency-comb spectrometer’, Opt. Lett., vol. 29, pp. 1542-1544 (2004), who suggested the use of a dual scanning laser system for Fourier Transform Spectroscopy (FTS) and the analysis of the spectral transmission of materials in the infrared spectral range.
In order to improve the scan rate of dual laser scanning FTS, Keilmann et al., in International Patent Application Publication WO2007/045461, further suggested to dither the repetition rate of one laser versus the other using techniques similar to the ones described in the '016 patent.
The use of lasers for spectroscopy has also been suggested by Haensch et al. in U.S. Pat. No. 7,203,402, where a single frequency comb laser based on a mode-locked laser was used for the measurement of certain properties of optical elements. Here the measurement was performed either simultaneously or sequentially at each individual frequency line of the comb laser.
A frequency comb laser was recently also combined with a conventional Fourier transform spectrometer to obtain an improved signal/noise ratio for spectral measurements (J. Mandon et al., ‘Fourier transform spectroscopy with a laser frequency comb’, in Nature Photonics, 2009)
Prior dual scanning laser systems have a number of limitations when applied to spectroscopy. The low repetition rate of implemented laser sources leads to excessively long data acquisition times. and the techniques for signal generation in the near IR to mid-IR spectral range are relatively cumbersome. Systems implemented with bulky solid-state lasers are not well suited for instrumentation applications and require a large components count. Other systems (P. Giaccari et al., ‘Active Fourier-transform spectroscopy combining the direct RF beating of two fiber-based mode-locked lasers with a novel referencing method’, Opt. Express., vol. 16, pp. 4347 (2008)) and (I. Coddington et al., “Coherent Multiheterodyne Spectroscopy Using Stabilized Optical Frequency Combs,” Phys. Rev. Lett. 100, 13902 (2008)) provide only very limited spectral coverage.